"Flanked" enemies are enemies that are shot from the side. To flank an enemy, have your allies in the mission distract them from the front, and then go around to the side and attack them. Alternatively, you can keep the enemies preoccupied by firing at them from the front and having your allies do the flanking and killing to also get this achievement.

After learning how to do scans, you are free to scan objects and enemies throughout the game. To do this, press B to bring up the Battle Focus wheel. Then just aim the center at it at objects or enemies and hold LT to start the scan.

The majority of things that will be scanned will be enemies. That being said, don't forget to scan objects and everything else to get this one faster.

This can be done very easily. In most areas that are owned by the aliens, you can find sticky grenades. These grenades stick to enemies, obviously. Get close to increase your accuracy with throwing the grenades.

Give 150 commands to agents while in Battle Focus within a single campaign

Obviously, you can just spam this, but I don't recommend doing that. Instead, wait until later in the campaign when your agents have multiple powers. You can order them to use all of their abilities all at once in the order that they are selected. This means you can get five or six commands in with a single Battle Focus, which makes grinding for this achievement much faster. In addition, since the grinding will be done later in the game, it is likely that you have already been issuing orders to them a lot anyway.

Complete an entire Operation without the player finding a weapon or using a grenade

This is actually not that hard. For one, it can be accomplished in the early missions of the game. Instead of shooting any aliens (it can't be done on the first missions as shooting aliens is a necessity to complete the first mission) yourself, just order your agents to do all of the killing and such for you.

When an agent's health is fully depleted, they will fall to the ground. There is then a set amount of time that you or another agent has to get to them and revive them before they are dead forever. Obviously, this is easily done on the easiest difficulty setting of Rookie, so if you are going for this achievement, go that route. As long as you make it a point to keep all of your agents healed, you should do fine.

There are four different classes in The Bureau: XCOM Declassified, and all four of the different classes are available to choose from as soon as you can start picking your missions from the map. Just alternate between them and you will be golden.

The Investigations are mini-side missions of sorts that only take place in the XCOM Headquarters. To gather them, just walk around the base and talk with everyone possible every time that you visit the base after and before a Major Operation takes place.

Agent customization can be accomplished either before a mission in the mission select screen, or during a mission at resupply drops. To customize an agent, you either need to change their first name, last name, or both. Then back out to get the achievement.

Use all of the agent and player abilities in the field within a single campaign

To get this one, you will need to complete every mission, as that is the only way you're going to be able to level yourself up all the way as well as all four different agents of four different classes. Only when everyone is leveled up all the way will they have all the abilities available to them. Then it's just a matter of making sure you use all of the abilities before the final level of the game.

In the second Major Operation of the game, the level ends when escaping on the Skyranger helicopter. However, you can choose to stay behind and continue engaging the alien enemies until they have all been defeated.

To do melee attacks, click in the right analog stick. Just run up to an alien and start hitting them repeatedly. Obviously, this is easier to do when you are doing it to the weaker aliens, such as Sectoids or Outsiders that don't have shields to protect them.

Strip all of the armor off of 5 Mutons before killing them within a single campaign

There are many, many Mutons encountered throughout the campaign, so this should be no problem. The armor of a Muton is represented by the outline of them that appears next to their name at the top of the screen, right by their health meter. When the Muton has full armor, the figure is all orange. Attacking each individual part of the Muton will drain their armor until it is all black, which then signifies that the Muton's armor has been completely destroyed.